This invention relates to a process for the production of aromatic carboxylic acids such as terephthalic acid, isophthalic acid, trimellitic acid, naphthalene dicarboxylic acid and benzoic acid.
Terephthalic acid, by way of an example, is an important intermediate for the production of polyester polymers which are used typically for fibre production and in the manufacture of bottles. Current state-of-the-art technology for the manufacture of terephthalic acid involves the liquid phase oxidation of paraxylene feedstock using molecular oxygen in a lower (e.g. C2–C6) aliphatic monocarboxylic acid, usually acetic acid, in the presence of a dissolved heavy metal catalyst system usually incorporating a promoter, such as bromine. Acetic acid is particularly useful as the solvent since it is relatively resistant to oxidation and increases the activity of the catalytic pathway. The reaction is carried out in a stirred vessel under elevated temperature and pressure conditions, typically 150 to 250° C. and 6 to 30 bara, respectively, and typically produces terephthalic acid in high yield, e.g. at least 95%.
Generally, however, the terephthalic acid obtained is not sufficiently pure for direct use in polyester production since it contains, as major impurities, partially-oxidised intermediates of terephthalic acid, particularly 4-carboxybenzaldehyde (4-CBA), along with various color-forming precursors and colored impurities. In a conventional process used for the production of terephthalic acid, a substantial proportion of the terephthalic acid tends to precipitate as it forms during the course of the reaction and, although it may be below its solubility limit in the solvent under the prevailing conditions, 4-CBA tends to co-precipitate with the terephthalic acid. This relatively crude terephthalic acid, therefore, has to be processed further to secure terephthalic acid of acceptable quality for use in production of high grade polyester. Such further processing typically comprises dissolving the impure terephthalic acid in water at an elevated temperature to produce a solution which is hydrogenated in the presence of a suitable catalyst, e.g. a noble metal catalyst on a carbon support. This hydrogenation step converts the 4-CBA to para-toluic acid while the various color bodies present in the relatively impure terephthalic acid are converted to colourless products. The purified terephthalic acid is then recovered from solution by a series of crystallisation, solid-liquid separation and drying steps. Because para-toluic acid is considerably more soluble in water than terephthalic acid, the former tends to remain in the aqueous mother liquor following crystallisation and solids-liquid separation. A process involving production of crude terephthalic acid and its subsequent purification by hydrogenation is disclosed in, for example, EP-A-0498591 and EP-A-0502628.
In a continuous process described in WO-A-98/38150, relatively high solvent/precursor ratios are employed, and, accordingly, substantially all of the aromatic carboxylic acid produced can be kept in solution thereby minimising co-precipitation of the reaction intermediates in the course of the reaction. As a result, the intermediates remain available for reaction to the desired aromatic carboxylic acid, and the rate of reaction is enhanced for the intermediates compared with a conventional process. By operating the oxidation reaction in this way, it is possible to reduce the extent of contamination of the aromatic carboxylic acid with any aldehyde produced as an intermediate in the course of the reaction. For instance, as mentioned above, in the case of terephthalic acid production by liquid phase oxidation of paraxylene or other precursor, the reaction results in the production of 4-carboxybenzaldehyde as an intermediate. Co-precipitation of 4-CBA with terephthalic acid is largely avoided since the terephthalic acid is not allowed to precipitate during the reaction, at least not to any substantial extent. Moreover, the conditions necessary to achieve this tend to lead to oxidation of intermediates such as 4-CBA to a greater extent to the desired end product.
Although, the process described in WO-A-98/38150 represents a valuable improvement over the prior art, it involves the use of substantial amounts of organic solvent. Although organic solvents, such as acetic acid, are particularly useful in such oxidation processes for the reasons given above, it would in certain situations be desirable to minimise their use. Such organic solvents are relatively costly and, due to environmental restrictions, may require recovery and recycling Furthermore, a proportion of the organic solvent may be ‘lost’ due to combustion during the oxidation reaction. A further problem with the use of acetic acid is that it is flammable when mixed with air or oxygen under typical reaction conditions in this system.
A further problem with the use of conventional solvents, such as acetic acid, is the low solubility of the oxidant component therein. Thus, where dioxygen is used as the oxidant, the dioxygen is present predominantly as discrete bubbles in the reaction medium with only a small proportion of the dioxygen dissolving in the solvent. To the extent that the reaction between the precursor and the dioxygen results from the dioxygen diffusing from the bubbles into the bulk liquid, the reaction rate is limited by the low solubility of dioxygen in the solvent.
Holliday R. L. et al (J. Superficial Fluids 12, 1998, 255–260) describe a batch process for the synthesis of, inter alia, aromatic carboxylic acids from alkyl aromatics in a reaction medium of sub-critical water using molecular oxygen as the oxidant. The dielectric constant of water decreases dramatically from a room temperature value of around 80C2/NM2 to a value of 5C2/NM2 as it approaches its critical point (374° C. and 220.9 bara), allowing it to solubilise organic molecules. As a consequence, water then behaves like an organic solvent to the extent that hydrocarbons, e.g. toluene, are completely miscible with the water under supercritical conditions or near supercritical conditions. Dioxygen is also highly soluble in sub- and super-critical water. The process described by Holliday et al was carried out in sealed autoclaves as a batch reaction.
It is an object of this invention to provide an alternative and improved continuous process for the production of an aromatic carboxylic acid, such as terephthalic acid, wherein substantially all of the aromatic carboxylic acid produced, i.e., intermediates and precursors, are maintained in solution during the reaction, and wherein the need to use an organic material, such as aliphatic monocarboxylic acid, as solvent is eliminated. It is a further object of this invention to provide an alternative and improved continuous process for the production of an aromatic carboxylic acid wherein substantially all of the reactants and product are maintained in a common phase during reaction. It is a further object of this invention to provide a continuous process, having good selectivity and high yield, for the production of an aromatic carboxylic acid by the oxidation of a precursor in sub- or super-critical water.
We have now devised a process which overcomes one or more of the problems previously encountered for the use of supercritical water.